The mining, construction, and drilling industries make extensive use of rotary tools. These tools apply intense loads to small areas to break brittle formations and structures into chips. Tapered cutting elements are commonly used for this purpose. Such elements are alternatively referred to as studs, buttons, cutters, cutting tools, and bits. A plurality of cutting elements may be attached to a holding tool. This tool may be a rotating drum, disc, or bit body. Such tools are used to mine minerals, cut trenches, mill pavement, drill holes, and the like.
A cutting element is a component of a cutting tool and is what contacts the formation being cut. The end portion of such a cutting element, which directly contacts the formation, is called the contact structure. Tapered cutting elements are well known, and they commonly have flat, rounded, or pointed contact structures. The principal, hardest, and most wear-resistant material of a contact structure is called a contact element, but adjacent parts of the end of the cutting element may also contact and help cut a formation, and not all contact structures have discrete contact elements. Cutting elements comprise at least a contact structure and a mounting structure. The mounting structure is used to mount and carry the contact structure, including any contact element. The mounting structure further comprises a holding structure and often a projection structure. The holding structure is the part of the mounting structure and the cutting element that is held in a cutting tool. When present, the projection structure is located between the contact structure and the holding structure. The projection structure distances the contact structure from the holding structure and thus from a surface of the cutting tool. The contact structure is then supported by the projection structure, and the projection structure is supported by the holding structure. When no projection structure is present, the mounting and holding structures merge together, and we speak of just the mounting structure as what carries the contact structure with respect to the cutting tool.
Typically, during use, only a portion of each contact structure is in contact with the formation at any given point in time. The size and the exact location of the area that is in contact with the formation depends on the design of the cutting element, the orientation of the cutting element with respect to the formation, the properties of the material being cut, and the operating conditions. Some portions of the contact structure are more likely than others to make contact with the formation being cut. The contact element is intended to be the first and principal point of contact with the formation, in normal use. Portions of the projection structure will rarely make contact with the formation being cut. Simple cutting elements may comprise only one part, made of one material, while more complex cutting elements may comprise more than one part made from several different materials, such as sintered tungsten carbide and steel. Usually the material of the contact element is harder and more resistant to abrasion than the material(s) of the projection structure and the mounting structure. The material(s) of the projection structure and mounting structure are often more ductile and resistant to impact than the material of the contact element. This selective use of materials in the prior art has improved performance of cutting elements while reducing their cost.
The properties, cost, and availability of sintered tungsten carbide make it the current material of choice for the majority of contact elements on most contact structures and cutting elements used in the construction, mining, and drilling industries. The sintered tungsten carbide used in such contact elements and structures is generally harder than Rockwell C 67. Many other materials are currently known that have similar properties to tungsten carbide but are not currently used to any significant degree. Some of the suitable materials are the oxides of metallic elements, the borides of metallic elements, the nitrides of metallic elements, the silicides of metallic elements, and the carbides of metallic elements. Steel is currently the material of choice for projection structures and for mounting structures, with hardness of less than Rockwell C 67. A harder surface treatment or coating may be applied to such projection and mounting structures.
FIG. 1 of the drawings shows a simple, prior art, tapered cutting element with a rounded contact structure 20, which has been in common use. It has a tapered mounting structure 22 and an adjoining holding structure 24. The contact structure comprises the integral rounded tip portion 20, without a separate contact element, and may include some or all of the tapered mounting structure 22. Any portion of the tapered structure that would only rarely contact the formation and is located between the contact structure 20 and the holding structure 24 is part of the projection structure. The included angle Φ1 of the distal end, taken from the tapered part below the rounded part, is acute. Cutting elements of this type are permanently installed. An interference fit between holding diameter D1 and the holding tool the usual method of retention. The ratio of the total length L1 to the holding diameter D1 is small, often approximately in the range of from 1 to 2. Simple cutting elements of this type are fabricated entirely from a single piece of sintered tungsten carbide.
FIG. 2 shows a more complex prior art cutting element that is also in common use in the art. It has two component parts, a contact element insert 26, which comprises much of the contact structure, and a mounting body 28, part of which is the holding structure 38 that is physically received inside a bit body. The holding structure 38 does not include a bit body socket rim bearing structure 36. The contact element 26 is shown assembled on the mounting body 28, on a common axis 30. The contact element 26 is part of the contact structure, and usually the contact element is made of tungsten carbide. Depending on the operating conditions and size of the contact element 26 in relation to an overall diameter D2, some of the contact structure may be located on the tapered portion of the mounting body 28, at 32, adjacent to the contact element 26. More rarely, some of the contact structure may be located on a portion of outside diameter D2. The projection structure on this cutting element normally includes the tapered face 32 of the mounting body 28 and the outside diameter D2 of this cutting element. The included angle Φ2 of the distal end of the cutting element is acute, as in FIG. 1. The distal end of the cutting element is shown as pointed, but it may sometimes be rounded. The mounting body 28 of the prior art cutting element of FIG. 2 is commonly made of steel.
The outside diameter D2 of the projection part of mounting body 28 in the FIG. 2 prior art device is larger than diameter D3 of an upper part of holding structure 38. A groove 34 is formed in the diameter D2 portion for engagement of a tool for removing the cutting element from a bit body. A bearing structure 36 is located at the base of the outside diameter D2. The holding structure 38 has two sections, one with the larger diameter D3 and another section with a smaller diameter D4. An additional groove 40 in the lower part of holding structure 38 facilitates engagement of a retainer for holding the cutting element rotatably in a bit body. The ratio of the overall length L2 of the cutting element to the largest holding structure diameter D3 is greater than 3.37 for known replaceable cutting elements.
Cutting elements used in the mining and construction industries are usually mounted so that they can be replaced when their contact structures become excessively worn. In the drilling industry, the vast majority of cutting elements now in use are not replaceable. Some can be sharpened. Usually, however, the entire drilling tool is replaced when the contact structures, contact elements, or cutting elements are significantly worn or damaged.
In the mining and construction industries, rotary tools are usually configured so that the cutting elements are rotatable about their long axes. The cutting elements and contact structures are angled backward from their direction of motion, and they also are angled to one side. These angulations cause each cutting element to rotate about its central axis when engaged, and so, as the sides of the contact structure wear, the original form of the contact structure is largely maintained.
A pointed end requires less force to initiate cracks in formations than other types of contact structure ends, because points best concentrate stresses. Such a pointed end also causes fewer unintended fractures in the material being cut than flat or rounded ends, making it the most energy-efficient type of cutter. Generally, in the mining and construction industries, large chips are desirable. Stresses are very high at the point of the contact structure, especially during impact with material being cut. The heat generated at the point is intense, and it may build up during times of continuous use. Abrasion at the point of a contact structure is much greater than on the flank of the contact structure, so the point can quickly become rounded. The point may also itself fracture. Designers have recognized these factors and have sometimes truncated the tapered end of the contact structure to create a cutting edge that is better supported and better cooled than a pointed contact structure. As the contact structure rotates in use, new portions of its cutting edge are presented to the material to be fractured.
In harder formations, nonetheless, pointed and truncated contact structures can be damaged so rapidly that they are impractical to maintain. This limitation has led to use of a radius or nose on the end of the contact element or structure, and the equipment employing such contact structures then applies higher forces. Several inventors have recognized these limitations, and patents have been issued for improvements in contact structures that help maintain the pointed form. Hard coatings have been applied as a means of extending the life of the point or the edge. Another approach has been to place a harder material in the center of the cutter that is supported by a softer, less wear resistant and more ductile material located radially outwardly from the center (U.S. Pat. No. 4,859,543). In these designs, both the harder and the more ductile materials have been made of sintered tungsten carbide with a cobalt binder. Tungsten carbides as hard as 88 Rockwell A have been used in these designs. Such designs have met with limited success, as the difference in hardness between the two grades of carbide used has not been great enough. Sintered tungsten carbide in grades as hard as 92 Rockwell A are readily available but are not known to have been used in either the center or outer structures. Contact stresses can fracture prior art contact structures.
Several patented bit designs use pointed contact structures. In one design, a number of angled replaceable cutting elements are rotatably attached to winged structures attached to a body (U.S. Pat. No. 5,735,360). Such elements are limited to larger bit sizes, and their use is limited to shallow holes in relatively soft formations. In another, somewhat similar design., a pilot cutter is located in the center of the bit (U.S. Pat. No. 3,720,273). The added center cutter gives this design better radial and axial stability than the pilotless type. These bits use a relatively compact cutting element compared to the cutting elements commonly used in mining machinery because of the limited space available on a bit. The numbers of cutting elements per unit of borehole area is relatively small, so the chips produced are relatively large. Typically for this type of bit the ratio of cutters divided by the cross sectional area of the bore has been approximately 0.2 cutters/square inch. At least one patented bit design uses pointed contact structures in rolling cones (U.S. Pat. No. 4,854,405). This design has not had been well accepted in the drilling industry, likely because in this type of bit the bearings wear out before the tungsten carbide cutting elements do.
Drag bits with flat contact structures having curved or straight cutting edges made of sintered tungsten carbide have been in use for many years. Within the last two decades, several new materials have been developed that can significantly improve the performance of these bits. The new materials are polycrystalline diamond and cubic boron nitride. The term polycrystalline is used to describe a multi-crystal composite material either with or without an additional material binding the individual crystals together. Polycrystalline materials can be fabricated into desired shapes and are much more resistant to impact damage than single diamond crystals. Polycrystalline diamond and cubic boron nitride are both substantially harder than impact grades of cemented tungsten carbide but are significantly less impact resistant. Of the two materials, polycrystalline diamond is the more commonly used material in the drilling industry. Contact structures of polycrystalline diamond have substantially increased the life of fixed cutting element drag bits, and they cut rock formations of increased hardness. The cost of polycrystalline diamond-coated contact structures is relatively high, and they are easily damaged and are generally not replaceable. At least one patent, however, appears for a replaceable diamond-coated contact structure (U.S. Pat. No. 4,782,903).
Polycrystalline diamond contact structures are bonded to tungsten carbide support structures to reduce the potential damage during use, to reduce cost, to facilitate processing, and to facilitate assembly. During use, polycrystalline diamond material has a tendency to delaminate from the tungsten carbide backing and to disintegrate. Many patents have been issued for improvements intended to reduce delamination and disintegration (e.g., U.S. Pat. No. 5,967,249). These bits are a vast improvement over bits that used single crystal cutting elements, which have been in use for over a century. These bits use the edges of the facets as cutting edges. Large numbers of diamonds are needed because the contact structures are small, they are irregular and the diamonds are brittle. As a result the chips cut from the formation face are very small, and chip flushing is poor. These bits cut very slowly and are currently used primarily for coring.
Several materials have been developed recently that show significant potential for use in contact structures. Two of these are carbon nitride and aluminum magnesium boride, but neither is available in such amounts as to presently allow their use commercially in contact elements.